When building antennas for the Wifi band (Like
the 8dBi omni), a need for an easy way to check the antennas arose. A
Voltage Standing Wave Ratio (VSWR) meter useable at the 2.4GHz band is
however, hard to find. Meters like the
Daiwa
CN-801S are expensive and need a minimum of 500mW (27dBm) power to
function correctly. Because of this, and because it is fun, this homebrew
project.

The circuit on this page serves as a Voltage Standing Wave
Ratio meter or VSWR meter for the
VHF-UHF-SHF frequency range. This includes the Wifi (802.11b/g), 2m, 70cm and
13cm Radio Amateur bands.

The device consists of two parts. One RF head
and a (preferably analog) indicator.
The picture to the rights shows the Wifi SWR meter Mk-III - the third
generation of the design.

An external SWR sensor is attached to the indicator with a BNC-BNC cable.

An Analog indicator is used to read the VSWR.
This meter has a Peak-Hold circuit with a 9 Volt
alkaline battery power supply. There is only one control to control the
power ON/OFF and meter Sensitivity. Meter sensitivity can be adjusted to
accommodate several RF sources, such as a Wifi Access Point, or VFO.

RF Input power must be somewhere between 15 and 20dBm (30 to 100mW)

Click on each image to enhance.

This is the diagram for the RF Head. It is
essential to use SMD size 0805 or smaller components and the PCB design below.

Zx is the antenna port.

“RF in” is connected to the Wifi Access Point
(or other RF Generator) port.

You may either use an etching process or a
sharp hobby knife to cut out the copper pattern on the PCB.
I always use the knife method, start with the 3mm center micro strip and work
from there.

Clear an area of copper behind the Zx port to minimise the
capacitance of this port to ground.

Mount the SMD Capacitors and resistors.
Mount
the diode last

The finished bridge
(Click for larger image)

Assemble the RF Head using the PCB assembly,
two flat N-Type bulkhead connectors, three metal 10mm long studs with M3
thread and six 7mm M3 machine screws.

Solder the PCB in place. Make sure that it is
not under mechanical stress. Solder the ground surface to the bulkhead
connector at 4 places, making as much contact as possible.

Solder the BNC to the assembly. The BNC is used to output the pulsed DC signal to the
indicator.

The photo shows three different versions of the bridge. RP-SMA, RP-TNC en
N-Type antenna connector.

Wifi hardware transmits in short bursts. You need a
“peak-hold” circuit to convert the DC pulses from the RF head
into a stable DC voltage for the indicating instrument. The design shown here
is based on a quad OPAMP, TL084.

OPAMP U1A creates a 'virtual ground' so that the OPAMP can be
powered by a single 9 Volt battery.

The voltage from the RF head consists of a pulsed DC signal (so
no 2.4GHz present). The pulse width varies between 0.2-0.5ms with a
repetition frequency equal to that of the SSID (beacon) interval set in the
AP. Voltage is a few volts max and depends on the amount of unbalance in the
bridge.

The diagram shows a Peak-Hold/ Fast switch. This switch is not required and
can be omitted.

Potentiometer SENSITIVITY is used to set the meter at full scale deflection
with a fully unbalanced bridge (short circuit plug attached to the Zx port).
The worst-case input VSWR of the bridge is 1:2, so you will NOT destroy your
Acces Point ;-)

OPAMP U1B forms a peak-hold circuit wit the general
purpose diode D1, C3 and R5. U1C and U1D buffer the output for the meter and
a second (normally unused) output. Output to the meter is limited to
+1.2/-0.6 Volt by means of three 1N4148 diodes.

Switch S2 can bypass diode D1. With S2 closed, the OPAMP U1b acts as a
unity gain buffer. The meter will react directly to changes in VSWR. This is
beneficial when the VSWR meter is used with an amateur band radio. If the
bridge is used exclusively on Wifi equipment, S2 can be omitted.

OPAMP U1D provides an adjustable voltage to the - terminal of the meter.
This allows compensation for leak-currents of D1 and offset voltage of the
OPAMPs.

Homebrew 2.4GHz dummy load:
Build a dummy load for 2.4GHz. This dummy load is
required to test and calibrate your new VSWR meter. Use a good quality
Female N-type bulkhead connector with flange, and two SMD 0805 or 1206 100Ω (1%) resistors. Solder the resistors directly to the
connector center contact, and use copper foil to connect the other side to
the connector ground. This prototype shows a VSWR of 1:1.01 @ 2.4GHz
compared to my reference (Narda Model 370 DC-18GHz dummy load) was used to
calibrate my bridge at 1:1.00. Maximum dissipation power is 0.5 Watt.

VSWR

P

1

0.000

1.1

0.048

1.2

0.091

1.3

0.130

1.4

0.167

1.5

0.200

1.6

0.231

1.7

0.259

1.8

0.286

1.9

0.310

2

0.333

2.1

0.355

2.2

0.375

2.3

0.394

2.4

0.412

2.5

0.429

2.6

0.444

2.7

0.459

2.8

0.474

2.9

0.487

3

0.500

4

0.600

5

0.667

6

0.714

7

0.750

8

0.778

9

0.800

10

0.818

Set

1.000

Manufacture a scale for the analog indicator as shown
in the table below. You will be able to directly read the VSWR from the
meter.
Indicator scale (P equals meter deflection, 1 = FSD):

Usage:
Attach the RF head to the Access Point or transmitter (maximum approximately
200mW) using suitable cable. Set the AP to transmit a beacon at an interval
of 100ms or less.
Attach a short circuit to the Zx port on the RF head. Adjust SENSITIVITY
control to have full
scale deflection on the meter.
Testing balance:
Attach a suitable 50Ω dummy load to the Zx port of the RF head. VSWR should
read close to 1:1.0.
Note: common VHF/UHF dummy loads do not perform well on this frequency!
Note: If required, BALANCE the bridge by adding or subtracting some
capacitance at the Zx port, or at the fixed leg of the bridge!
You can reduce capacitance by cutting away copper foil. Add capacitance by
soldering copper vanes to the ground plane near 'hot' tracks.
Testing unbalance:
Attach a known mismatch dummy load to the Zx port of the RF head. VSWR
should be indicated correctly.
Determine antenna VSWR:
Attach the antenna under test to the Zx port of the RF head. Position the RF
head as close as possible to the antenna under test, using as little coax
cable as possible.
Read the VSWR from the indicator.

I will not be responsible for damage to equipment, your ego, personal injury or worse that may result from the use of this material and material found on any links on my pages. You are responsible to make sure that your use any of my designs is legal in your country.